Synthesis of Ethyl-4-ethoxy Pentanoate by Reductive Etherification of Ethyl Levulinate in Ethanol on Pd/SiO2-C Catalysts

Article abstract of DOI:10.1002/cssc.201801624

The synthesis of biomass-derived ethers to be used as biofuels or biofuel additives has attracted much attention. Following the recently reported synthesis of etherified ester ethyl-4-ethoxy pentanoate (EEP) from γ-valerolactone (GVL) in ethanol catalyzed by H-beta zeolite, an alternative route to prepare EEP in high yield has been developed by reductive etherification of ethyl levulinate (EL) in ethanol at 140 °C under 0.5 MPa H₂ with a silica-modified Pd/C catalyst. The ether production likely follows a tandem acetalization–hydrogenolysis process with ethyl-4,4-diethoxy pentanoate (EDEP) as the intermediate. The acetalization step can be favored by introducing acidic materials, such as SiO₂–carbon or beta zeolite, as a cocatalyst. The combination of the Pd/SiO₂-C and beta zeolite mixture leads to 100 % EL conversion and 93 % EEP selectivity under optimized reaction conditions. For the first time, the standard molar combustion enthalpy of as-prepared EEP is measured by using a static oxygen bomb, and the value of which is determined to be about ?5658 kJ mol^?1, which is much larger than that of GVL (?2650 kJ mol^?1).

Full text of DOI:10.1002/cssc.201801624

Accepted Article
Title: A facile route toward the synthesis of ethyl-4-ethoxy pentanoate
in high yield: reductive etherification of ethyl levulinate in ethanol
on Pd/SiO2-C catalysts
Authors: Qianqian Cui, Yinshuang Long, Yun Wang, Haihong Wu,
Yejun Guan, and Peng Wu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201801624
Link to VoR: http://dx.doi.org/10.1002/cssc.201801624
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10.1002/cssc.201801624
ChemSusChem
FULL PAPER
A facile route toward the synthesis of ethyl-4-ethoxy pentanoate
in high yield: reductive etherification of ethyl levulinate in ethanol
on Pd/SiO2-C catalysts
Qianqian Cuia, Yinshuang Longa, Yun Wanga, Haihong Wua, Yejun Guana*, Peng Wua
Abstract: Facile synthesis of biomass-derived ethers useful as
biofuels or biofuel-additives has attracted great attention. We have
recently reported the synthesis of a novel etherified ester ethyl-4-
ethoxy pentanoate (EEP) from gamma-valerolactone (GVL) in ethanol
catalyzed by H-Beta zeolite. In this continuous study, we report an
alternative route to prepare EEP in high yield by reductive
etherification of ethyl levulinate (EL) in ethanol at 140 °C under 0.5
MPa H2 with silica modified Pd/C catalyst. The ether production likely
follows a tandem acetalization-hydrogenolysis process with ethyl-4,4-
diethoxy pentanoate (EDEP) as intermediate. The acetalization step
can be favored by introducing acidic materials, SiO2-carbon or Beta
zeolite as a cocatalyst. The combination of Pd/SiO2-C and Beta
mixture leads to 100% EL conversion and 93% EEP selectivity under
optimized reaction conditions. We have for the first time measured the
standard molar combustion enthalpy of as-prepared EEP by static
oxygen-bomb, and the value of which is determined to be about -5658
kJ/mol, much larger than that of EL (-2650 kJ/mol).
synthesis of bio-ethers such as 5-ethoxymethylfurfural13, 14 and
glycerol ethers15, 16 via acid-catalyzed etherification of biomass-
derived alcohols has attracted great attention. We have recently
reported the synthesis of a novel bio-derived etherified ester,
ethyl-4-ethoxy pentanoate (EEP), by simply heating GVL in
ethanol in the presence of solid-acid catalyst of Beta zeolite.17 The
EEP yield and selectivity reached 86% and 97%, respectively at
reaction temperature of 160 °C. However, the EEP yield could not
be further increased because (a) the GVL conversion is limited by
the reaction equilibrium between GVL and ethyl-4-hydroxyl
pentanoate (EHP), and (b) several dehydration by-products such
as ethyl pentenates were formed at higher reaction temperatures
(Scheme S1). Therefore, developing alternative route is desirable
in the viewpoint of green and sustainable chemistry.
Reductive etherification of carbonyl compounds provides an
alternative methodology for the preparation of biomass-derived
ethers.18-20 Balakrishnan et al.18 have reported the reductive
etherification of 5-(hydroxymethyl)furfural catalyzed by supported
Pt/alumina catalyst in presence of Amberlyst-15. This method
also applies to furfural in both methanol and ethanol by using
carbon-supported noble metal nanoparticles as catalyst, though
under different reaction conditions.19, 20 We have found that Pd/C
can efficiently catalyze the reductive etherification of furfural in
ethanol under mild conditions (0.3 MPa H2 and 60 oC).20
Tulchinsky and Briggs recently reported the synthesis of alkyl 4-
alkoxypentanoates by simultaneous esterification and reductive
etherification by using levulinic acid as precursor, where Pd/C was
used as catalyst under hydrogen pressure 100-700 psig at 200-
220 oC.21 However, the yield of etherified esters were limited due
to the high temperature used.21 The results in this work
demonstrate that Pd/C shows moderate activity for EL conversion
but high selectivity toward EEP via reductive etherification. The
EL conversion over Pd/C can be enhanced by modifying
amorphous SiO2 onto the carbon surface. Moreover, the EEP
yield was further increased to 93% when Beta was introduced as
a cocatalyst at 140 oC. The EEP formation most likely undergoes
a two-step (acetalization and hydrogenolysis) process. The
introduction of amorphous SiO2 and Beta accelerates the
acetalization owing to their acidic nature. The combustion value
of as-prepared EEP has been determined in a static-bomb
thermometer. This work gives another example for preparation of
bio-derived etherified ester from carbonyl compounds via
reductive etherification.
Introduction
As the worldwide supply of petroleum diminishes, it is becoming
increasingly important to produce both chemicals and
transportation fuels economically from biomass that can be
utilized as a renewable carbon source.1-6 Three classes of
feedstocks derived from biomass are considered to be
appropriate for the production of renewable fuels: starchy
(including sugars), triglyceride, and lignocellulosics.7 Among
these feedstocks, lignocellulosic biomass is the most inexpensive
and abundant class of renewable, which can be converted to so-
called platform chemicals such as furfural, alkyl levulinate, and
8-10
gamma-valerolactone (GVL).3,
This methodology affords a
greater degree of flexibility in downstream processing of biomass
derived oxygenates such as ethanol, furan, ester, acetal, and
ether. More importantly, adding oxygenated additives into
transportation fuel can remarkably improve combustion and
reduce the emissions of some regulated pollutants originating
from transportation, which allows the clean air standards to be
met.11, 12 In this regard, bio-derived ethers have been identified as
a representative category of valuable fuel-additives.2 Therefore
[a]
Ms. Qianqian Cui, Ms. Yinshuang Long, Ms. Yun Wang, Prof.
Haihong Wu, Dr. Yejun Guan, Prof. Peng Wu.
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, School of Chemistry and Molecular Engineering, East
China Normal University, North Zhongshan Road 3663, Shanghai,
China
Results and Discussion
Effect of temperature, Pd loading, and hydrogen pressure on
the reductive etherification of EL over Pd/C catalyst
E-mail: yjguan@chem.ecnu.edu.cn
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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As an initial study, we tested the reductive etherification of EL over
2.1 wt% Pd/C catalyst at reaction temperatures ranging from 50
to 140 oC under 0.5 MPa H2 and the results are shown in Figure
1. The 2.1 wt% Pd/C catalyst was active for EL hydrogenation at
50 oC in ethanol, giving EL conversion of 16% after reaction for 4
h. The product selectivities are as follows: 73% to EEP, 20% to
EHP, 7% to GVL. To our delight, the formation of etherified ester,
EEP, was readily observed at temperature as low as 50 oC.
Further increase of temperature led to the increase of both EL
conversion and EEP selectivity. When the reaction temperature
was 140 oC, the EL conversion and EEP selectivity were 60% and
90%, respectively. It should be noted that the hydrogenation of EL
in ethanol over 2.0 wt% Ru/C catalyst at 140 oC gives GVL as the
main product with a selectivity as high as 98% (Table S1). This
result points to the specific role of Pd nanoparticles in reductive-
etherification.
dominating product for all catalysts. The by-products were found
to be ethyl valerate (EV) and GVL. When the Pd loading was 3.1
wt%, EL conversion reached 75% and the selectivity of EEP was
as high as 90%. We further investigated the effect of H2 pressure
on the reductive etherification over 2.1 wt% Pd/C catalyst. The
results shown in Table 1 (Entries 4, 6, and 7) suggest that the
higher H2 pressure benefits the EL conversion, meanwhile, the
EEP selectivity remains above 90%. The EL conversion reached
82% under H2 pressure of 1 MPa with EEP selectivity of 91%.
Effect of modifying inorganic oxides on the catalytic activity
of Pd/C catalyst
The above-mentioned results demonstrate that Pd/C catalyst
shows moderate activity but high selectivity toward EEP in
reductive etherification of EL. It has been shown recently that the
activity and stability of carbon supported metal nanoparticles can
be remarkably increased by modifying inorganic oxides.22-28
Therefore, we have prepared several inorganic oxide and carbon
composites (MOx-C) by introducing SiO2, Al2O3, TiO2, CeO2, ZrO2,
and Nb2O5 onto C surface via impregnation or hydrolysis methods
with MOx loading about 20 wt%. The catalytic activity of Pd
supported on these supports are listed in Table 2.
100
80
60
EL Conv.
40
20
0
EEP
EV
GVL
EHP
Table 2. Catalytic Activity of Pd/C modified with Various Oxides
(20 wt%) for the Reductive Etherification of Ethyl Levulinate (EL).
Sel. (%)
Conv.
(%)
17
25
33
28
45
65
Catalysts
EV
11
2
10
5
EEP
75
26
GVL
14
62
4
EHP
40
60
80
100
120
140
2.6Pd/Al2O3-C
2.7Pd/CeO2-C
2.5Pd/Nb2O5-C
2.5Pd/ZrO2-C
2.5Pd/TiO2-C
2.5Pd/SiO2-C
-
10
-
Temperature (oC)
86
Figure 1. Effect of reaction temperature on the catalytic activity of
2.1 wt% Pd/C catalyst for the reductive etherification of ethyl
levulinate (EL). Reaction conditions: 350 µL of EL in 9.65 mL of
ethanol; 0.5 MPa H2; 50 mg of 2.1Pd/C catalyst; 4 h. EV: ethyl
valerate; GVL: gamma-valerolate; EHP: ethy-4-hydroxyl
pentanoate; EEP: ethyl-4-ethoxy pentanoate.
85
91
6
6
4
-
3
1
93
6
-
Reaction conditions: 350 µL of EL in 9.65 mL of ethanol; 140 °C;
50 mg of Pd catalyst; 0.5 MPa H2; 4 h. EV: ethyl valerate; GVL:
gamma-valerolate; EEP: ethyl-4-ethoxy pentanoate.
Table 1. Effects of Pd Loading, H2 Pressure on the Reductive
Etherification of Ethyl Levulinate (EL) over Pd/C Catalyst.
One can clearly see that Pd/C catalyst modified with different
oxides showed distinct activities. The 2.6Pd/Al2O3-C catalyst only
gave 17% EL conversion with EEP selectivity of 75% and the
byproducts mainly included EV and GVL. The catalysts modified
with CeO2, ZrO2, Nb2O5, and TiO2 gave EL conversion of 25%,
28%, 33%, and 45%, respectively. These catalysts all showed
much lower activity than Pd/C catalyst (60%). It should also be
noted that the major product over Pd/CeO2-C was GVL instead of
EEP. Different to these metal oxides based Pd/MOx-C, Pd/SiO2-
C showed slightly higher activity (65%) than Pd/C (60%). Inspired
by this result, we then compared the catalytic activity of Pd/SiO2-
C with varying SiO2 content and the results are shown in Fig. 2.
When 2.5 wt% SiO2 was loaded, the EL conversion was about
68%. Further increase of SiO2 led to higher EL conversion. A
maximum EL conversion about 73% was achieved for both
Pd/5SiO2-C and Pd/10SiO2-C. By contrast, Pd/SiO2 only gave EL
conversion and EEP selectivity of 4% and 57%, respectively. This
finding suggests that modifying SiO2 onto the carbon surface may
affect the catalytic activity of SiO2-C composite as well as the
Pd/SiO2-C catalyst. It can be concluded that the modified oxides
H2
Sel. (%)
Conv.
(%)
Entry Catalyst
Pressure
(MPa)
0.5
0.5
0.5
0.5
0.5
0.3
1
EEP
EV
GVL
1
2
3
4
5
6
7
0.5Pd/C
1.0Pd/C
1.5Pd/C
2.1Pd/C
3.1Pd/C
2.1Pd/C
2.1Pd/C
16
22
42
60
75
37
82
70
86
92
90
92
90
91
30
9
4
5
3
0
5
4
5
5
6
8
2
1
Reaction conditions: 350 µL of EL in 9.65 mL of ethanol; 140 °C;
50 mg of Pd catalyst; 4 h. EV: ethyl valerate; GVL: gamma-
valerolate; EEP: ethyl-4-ethoxy pentanoate.
We next explored the effect of Pd loading on the etherification
activity of Pd/C catalyst at 140 oC and 0.5 MPa H2 and the results
are displayed in Table 1 (Entries 1-5). One can see that as the Pd
loading increases from 0.5 wt% to 3.1%, the EL conversion
increases from 16% to 75%. EEP was observed as the
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10.1002/cssc.201801624
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influence the catalytic performance and product distribution. We
next followed the EL conversion and product distributions during
the reaction course by choosing Pd/C, Pd/CeO2-C, and Pd/SiO2-
C as examples and the results are shown in Tables S2-S4,
respectively. One can see that in the early stage of the reaction
(10 min), the acetalization product, ethyl 4,4-diethyoxypentanoate
(EDEP), was detected with varying selectivity ranging from 10 to
26% depending on the nature of support. This phenomenon
suggests that acetalization of EL as shown in Equation 1 takes
place on all catalysts in the beginning of the reaction.
of EDEP is rather low during the reductive-etherification process
of EL. To clarify the role of EDEP in the production of EEP, we
have investigated the hydrogenation activity of EDEP without
catalyst or in presence of Pd catalysts (Table S5). The EDEP was
1
synthesized according to a previous report33. The H-NMR and
GC-MS spectra is consistent with literature33, 34 and its purity was
determined to be about 94% (Figure S1). Upon heating at 140 oC,
the EDEP turns back to EL in ethanol with 100% conversion and
100% selectivity without any catalyst, meaning that the EDEP is
not stable in ethanol under the reaction conditions. This is in line
with the fact that the synthesis of EDEP in high yield requires the
usage of triethyl orthoformate as reactant, which acts as a
dehydration reagent33.When Pd catalysts were present, the
conversions were 97% and EEP was observed as one of the
major product with selectivity ranging from 15 to 35%. The EEP
selectivity over Pd/C, Pd/CeO2-C and Pd/SiO2-C are 23%, 15%,
and 35%, respectively. This result clearly indicates that the EEP
can be produced by hydrogenolysis of EDEP. Moreover,
modifying SiO2 onto carbon significantly improves the EEP yield.
The positive role of SiO2 might be two-fold. First, the acetalization
is likely catalyzed by either the Pd-H species on Pd/C catalyst20
or by Brønsted acid sites when solid acidic materials such as
Amberlyst-15 were present18. Second, the acidic sites catalyze
the protonation of acetal leading to the formation of enolether
intermediate, which is eventually hydrogenated to ether.32 In this
regard, introducing SiO2 increases the amount of acidic sites of
carbon material, e.g. lactonlic and phenolic groups, according to
a recent study by Gatti and coworkers27.
In accordance with this assumption, we then tested the catalytic
activity of Pd/xSiO2-C in presence of 25 mg of Beta zeolite for EL
hydrogenation and the results are shown in Table 3 and Table S6.
One can see that the EL conversions over Pd/C, Pd/SiO2 and
Pd/xSiO2-C are all significantly improved. As an example, EL
conversion increased from 60% to 87% over 2.1Pd/C in the
presence of Beta. For Pd/SiO2, the EL conversion also increased
from 4% to 16%. It should be noted that for the combinations of
Pd/xSiO2-C and Beta, the 2.5Pd/20SiO2-C and Beta showed the
highest EL conversion of 95%, with EEP yield of 88% at 140 oC.
This value is much higher than that obtained (55%) for Beta-
catalyzed ring-opening of GVL in ethanol at the same
temperature17. These results agree with the above-mentioned
assumption that acidity favors the reductive-etherification. We
tested the catalytic performance of 2.5Pd/20SiO2-C in the
presence of other commonly used zeolites, namely, USY
(SAR=25) and ZSM-5 (SAR=58). The results in Table S7
demonstrate that USY and ZSM-5 also showed promotion effect
but not as pronounced as that observed in case of Beta. The EL
conversion over USY and ZSM-5 were 74 and 79% under the
same reaction conditions, respectively. The total acidity of three
zeolites is as follow: Beta (2.2 mmol/g)  ZSM-5 (1.8 mmol/g) 
USY-6 (0.45 mmol/g)17, which is in good agreement with the trend
of EL conversions. Therefore, we consider that it is highly likely
that the acidic zeolites are involved in the reductive etherification.
It should be pointed out that the intimate contact between Pd
nanoparticles and acidic sites of zeolites is not necessary since
Pd/20SiO2-C and zeolites were physically mixed. Another
observation should be noted is that lower EEP selectivity was
EL + 2C2H5OH ⇌ EDEP +H2O
-------------------Equation 1
100
80
60
40
20
0
//
//
EL Conv.
EEP
EV
GVL
//
Pd/C
//
30
10
Pd/x-SiO2-C (x: wt%)
Pd/SiO2
2.5
5
20
Figure 2. Effect of SiO2 loading on the catalytic performance of
2.1 wt% Pd/C, 3.0 wt% Pd/SiO2, and 2.5 wt% Pd/xSiO2-C (x=2.5,
5, 10, 20, and 30 wt%) catalysts in reductive etherification of EL.
Reaction conditions: 350 µL of EL in 9.65 mL of ethanol; 140 °C;
50 mg of catalyst; 0.5 MPa H2; 4 h. EV: ethyl valerate; GVL:
gamma-valerolate; EEP: ethyl-4-ethoxy pentanoate.
It should also be noted that the selectivity to EHP at 10 min for
Pd/CeO2-C catalyst was about 62%, while minute or very little
amount of EHP was observed for Pd/C and Pd/SiO2-C catalyst.
This result means that Pd nanoparticles on CeO2-C are more
active toward saturation of carbonyl bond, thus forming large
amount of EHP. As a result, the lactonization of EHP will
undoubtedly lead to the formation of GVL, which explains the low
EEP yield on Pd/CeO2-C catalyst. This phenomenon is consistent
with a previous report that Pd/CeO2 is an active catalyst for GVL
production.29
The mechanism of reductive-etherification of EL
The above-mentioned results demonstrate that EEP can be
successfully synthesized via reductive-etherification of EL on
Pd/C modified with various oxides, though with different activity
and selectivity. The reductive-etherification of carbonyl
compounds generally undergoes a consecutive acetalization-
hydrogenolysis pathway.30-32 For instance, acetal of
cyclohexanone, octanal and furfural have been observed in large
amount without exceptionally in the early stage of their reductive-
etherification process. Taking furfural as an example, it is highly
likely that most furfural was first converted to furfural acetal, which
is then converted to furfuryl ethyl ether. By contrast, the amount
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10.1002/cssc.201801624
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noticed after introducing USY and ZSM due to the formation of
EV. This result is consistent with our previous finding that strong
Brønsted acid sites contribute to the production of EV.35
Bio-ether has been considered as potential fuel-additive. In this
regard, energy content is one of the most important features
accounting for its practical application. Therefore, we have
prepared about 2 g of crude EEP with purity close to 95% after
evaporating the solvent and trace amount of EV. Figure S2 shows
1
Table 3. The Catalytic Performance of Pd/C, Pd/SiO2, and
Pd/xSiO2-C Catalysts in Presence of 25 mg of Beta Zeolite.
Sel. (%)
the H NMR spectroscopy of the crude product, which confirms
the chemical structure of EEP molecule. We next tested its
standard (po=0.1 MPa) molar enthalpy of combustion in oxygen at
Catalyst
Conv. (%)
25 C (cHmo) by a static oxygen-bomb calorimetry with benzoic
o
EV
12
6
21
7
3
5
8
EEP
85
89
77
89
93
91
87
GVL
3
5
2
4
4
4
5
o
acid as reference. The cHm of EEP is estimated to be -5658
2.1Pd/C
87
88
82
90
95
88
16
kJ/mol, which is much larger than that of GVL (-2650 kJ/mol).37
More importantly, the density of EEP determined at 20 oC by
measuring the weight of 1 mL of EEP is found to be about 1.239
g/mL, suggesting that EEP has high-volume energy density. The
miscibility is another factor regarding to its application as fuel-
additive. Figure S4 shows the solubility of 2 mL of EL, GVL, EEP,
and MTBE with 6 mL of n-heptane. One can see that a
homogenous solution was obtained when EEP or MTBE was
mixed with n-heptane, whereas this is not the case for EL and
GVL, which means that the etherification strongly increases the
solubility of ester in hydrocarbon. The boiling point of EEP has
also been determined under 1 atm and it is found to be about
1382 oC.
2.5Pd/2.5SiO2-C
2.5Pd/5SiO2-C
2.5Pd/10SiO2-C
2.5Pd/20SiO2-C
2.5Pd/30SiO2-C
3.0Pd/SiO2
Reaction conditions: 350 µL of EL in 9.65 mL of ethanol; 140 °C;
50 mg of Pd catalyst; 25 mg of Beta (SAR=25); 0.5 MPa H2; 4 h.
EV: ethyl valerate; GVL: gamma-valerolate; EEP: ethyl-4-ethoxy
pentanoate.
We next studied the effect of Beta loading on the reductive
etherification activity of Pd/20SiO2-C catalyst and the results are
shown in Table 4. It can be seen that the EL conversion increased
to 77% when 5 mg of Beta was added in the reaction mixture while
the EEP selectivity was not changed significantly. The EL
conversion further increased to about 91% when the amount of
Beta was 15 mg. The addition of more amount of Beta did not
significantly change the EL conversion. With respect to the role of
Beta in this reaction, it is worthy of noting that Verhoef and
coworkers36 have found that H-MCM-22 itself may catalyze the
reductive etherification of substituted cyclohexanones with
secondary alcohols. They suggested that the protons or Lewis
sites in zeolites might be able to catalyze this reaction. However,
our control experiment showed that Beta alone did not show
catalytic activity in converting EL to EEP. Moreover, the 2.5 wt%
Pd/Beta catalyst also gave very low EL conversion of 14% under
the identical reaction conditions. Therefore we believe that Beta
serves as cocatalyst of Pd/xSiO2-C for this process, likely
enhancing the forward reaction of acetalization and the
protonation of acetal intermediate. Moreover, increasing H2
pressure to 1 MPa led to almost complete conversion of EL and
the EEP yield could reach 93%. Provided the excellent
performance of the 2.5Pd/20SiO2-C and Beta mixture, we have
further explored their reusability for the reductive etherification.
Results shown in Figure 3 suggest that after four sequential
reactions, the EL conversion dropped slightly from 90% to 80%,
while the EEP selectivity attained above 95%. This result means
that the reductive etherification pathway is very promising
compared with the acid-catalyzed etherification that undergoes
deactivation due to the coke formation.17 The spent catalyst was
characterized by TEM analysis and the result is shown in Figure
S3. One can see that the aggregation of Pd nanoparticles
occurred during the hydrogenation process, which accounts for
the decrease of EL conversion.
Table 4. Catalytic Activity of 2.5 wt% Pd/20SiO2-C in Presence of
Various Amount of Beta zeolite for the Reductive Etherification of
EL.
Beta
loading
(mg)
-
5
15
Sel. (%)
Catalysts
Conv. (%)
EV EEP GVL
2.5Pd/Beta
14
77
91
95
91
>99
3
2
3
3
4
3
97
94
93
93
92
93
0
4
4
4
4
4
2.5Pd/20SiO2-C
2.5Pd/20SiO2-C
2.5Pd/20SiO2-C
2.5Pd/20SiO2-C
25
100
2.5Pd/20SiO2-C a 25
Reaction conditions: 350 µL of EL in 9.65 mL of ethanol; 0.5 MPa
H2; 9.65 mL ethanol, 140 °C; 50 mg of Pd/20SiO2-C catalyst; 4 h.
a 1 MPa H2.
100
80
60
40
20
0
100
80
60
40
20
0
Fresh
3rd
1st
2nd
Figure 3. The reusability of 2.5wt% Pd/20SiO2-C and Beta mixture
for the reductive etherification of EL. Reaction conditions: 350 µL
The combustion value and miscibility of EEP
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1000
800
600
400
200
0
of EL aqueous solution; 0.5 MPa H2, 9.65 mL ethanol; 140 °C; 50
mg of Pd/20SiO2-C catalyst and 25 mg of Beta; 4 h.
A
B
+ 100
Pd/C
BET, XRD, CO chemisorption and TEM characterization of
supported Pd catalysts
Pd/C
Pd/20SiO2-C
Pd/SiO2
Pd/20SiO2-C
Pd/SiO2
The above results have shown that supporting SiO2, a well-known
inert material, onto Pd/C improves the catalytic activity in
reductive etherification of EL to EEP. We then characterized the
Pd/C, Pd/SiO2, and Pd/20SiO2-C catalyst and the major textural
properties are listed in Table 5. The XRD patterns of Pd/C with
different Pd loading, Pd/MOx-C and Pd/xSiO2-C catalysts are also
shown in Figures S5-7.
0.0
0.2
0.4
0.6
0.8
1.0
10
20
30
40
50
60
Relative Pressure (P/P0)
2 Theta (degree)
Table 5. Chemical Composition, Textural and Structural
Properties of Various Catalysts.
Pd
(wt.%)a
SSAb
(m2 /g)
Vtotal
(cm3/g) (%)
Dc
Sized
(nm)
Catalyst
Figure 4. N2 adsorption/desorption isotherms (A), XRD patterns
(B) and TEM images (C) of Pd/C, Pd/20SiO2-C, and Pd/SiO2
catalysts.
Pd/SiO2
Pd/C
3.1
2.1
209
1.34
0.88
0.89
4.8
40
47
7.2
6.3
5.0
1297
1330
Pd/20SiO2-C 2.5
Conclusions
a Pd loading Determined by ICP.
b Specific surface area Calculated using BET method.
c Dispersion estimated by CO chemisorption.
In summary, we have demonstrated another example of
preparing etherified ester via reductive etherification of biomass-
derived platform compound containing carbonyl group. With ethyl
levulinate (EL) as starting reagent, ethyl-4-ethoxy pentanoate
(EEP) can be synthesized with yield about 75% in ethanol at
140 °C for 4 h under 1 MPa H2 over Pd/C catalyst. The EEP yield
can be significantly improved to 93% by introducing amorphous
SiO2 onto carbon surface and adding Beta as cocatalyst in the
reaction mixture under mild conditions. It is likely that the acidity
of SiO2/C and Beta is beneficial to the reductive etherification,
possibly enhancing the forward reaction of acetalization
equilibrium. The standard molar combustion enthalpy of as-
prepared crude EEP with purity about 95% is determined to be -
5658 kJ/mol, which suggests that EEP may serve as a fuel-
additive with great potential.
d Average particle size given by TEM investigation.
All Pd catalysts show dispersion higher than 34% except for
Pd/SiO2 (4.8%) and Pd/Beta (4.0%). Figure 4A shows the N2
adsorption/desorption isotherms of Pd/C, Pd/SiO2, and Pd/SiO2-
C catalysts. Pd/C and Pd/SiO2-C show very similar surface area
of about 1300 m2/g, while Pd/SiO2 shows the lowest surface area
of 209 m2/g. The powder X-ray diffraction patterns in Figure 4B
suggest that the diffraction peak at 40o assigned to Pd
nanoparticles is clearly observed for Pd/SiO2, while no obvious
diffractions of Pd are found for Pd/C and Pd/SiO2-C. We next
determined the Pd particle size by TEM analysis. As shown in
Figure 4C, the average Pd particle size of Pd/SiO2, Pd/C, and
Pd/SiO2-C is estimated to be 7.2, 6.3, and 5.0 nm, respectively.
The dispersion of these Pd nanoparticles was also determined by
CO chemisorption (Table S8). Pd/20SiO2-C shows the highest
dispersion of 47%, while Pd/SiO2 shows the lowest dispersion of
4.8%. This trend is in line with the TEM analysis and explains the
highest activity of Pd/SiO2-C in EL reductive etherification. This
result also gives another example in emphasizing the advantages
of oxide-carbon composites their application in catalysis,
especially in liquid phase hydrogenation. The presence of carbon
ensures the high dispersion of Pd nanoparticles and meanwhile,
the oxides tune the acidic properties of the catalyst. These
composites are expected to find wide application in biomass
conversion.38
Experimental Section
Materials synthesis
Activated carbon (C) was obtained from STREM Chemicals with a surface
area about 1400 m2/g. The SiO2-C composites with different SiO2 loadings
ranging from 0 to 30 wt% were prepared by hydrolysis of tetraethyl
orthosilicate (TEOS). In a typical synthesis, 1 g of activated carbon was
dispersed in 30 mL of ethanol and stirred for one hour. Desired amount of
TEOS was added into the mixture dropwise and the resulted solution was
stirred overnight allowing for the complete hydrolysis of TEOS. The slurry
was filtered, washed with deionized water, dried at 60 °C for 12 h, and then
calcined at 500 °C for 3 h with a ramp rate of 3 °C/min under a N2 flow.
The sample was denoted as xSiO2-C (x stands for the silica loading in
wt.%). For comparison, C supported metal oxides, namely Al2O3-C, CeO2-
C, ZrO2-C and Nb2O5-C, have also been prepared by incipient
impregnation with the corresponding metal salts (Al(NO3)39H2O,
Ce(NO3)36H2O, ZrOCl26H2O, Niobium(V) oxalate hydrate). The
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10.1002/cssc.201801624
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preparation of TiO2-C can be referred to our previous study39, except that
tetrabutyl titanate was used as Ti precursor in this work. Beta (SAR=25,
646 m2/g) and ZSM-5 (SAR=58, 401 m2/g) were supplied from Nankai
Catalyst Company, China. USY-6 (SAR=24, 623 m2/g) was offered by
Huahua Catalyst Manufacturing Company, China. The detailed textural
and acidic properties of three zeolites can be referred to our previous
work.17
To shed light on the reaction mechanism of reductive-etherification of EL,
the possible intermediate, ethyl 4,4-diethyoxypentanoate (EDEP), was
synthesized according to a previous report32. In a typical synthesis, to a
solution of EL (2.84 mL, 20 mmol) in absolute ethanol (8 mL) were added
triethyl orthoformate (3.29 mL, 20 mmol) and 10 mg of p-toluenesulfonic
acid. The solution was heated at 145 oC for 2 h. After neutralization with
NEt3 and evaporation of the solvent, EDEP was obtained as a pale-yellow
oil with yield about 94%. The 1H-NMR and GC-MS spectra of the obtained
EDEP are in agreement with the reported data (Figure S2).33, 34
Preparation of supported Pd catalysts
The Pd catalysts with Pd loading of 0~3 wt% were prepared via a
40
deposition–precipitation method.39,
In a typical synthesis, 0.3 g of
Acknowledgements
support was distributed in 75 ml of H2O with vigorous stirring for 2 h. An
appropriate amount of H2PdCl4 aqueous solution was added into the
mixture. After being stirred for another 2 h, the final pH value of the mixture
was adjusted to 10~10.5 by adding NaOH solution (1 M). Then, NaBH4
aqueous solution (NaBH4/Pd = 15, molar ratio) was added into the
suspension to reduce the Pd2+. The slurry was stirred for another 2 h,
filtered, washed with deionized water, and finally dried at 60 °C for 12 h.
The supports used in this study included SiO2-C, Al2O3-C, TiO2-C, CeO2-
C, ZrO2-C, and Nb2O5-C.
This work was financially supported by the China Ministry of
Science and Technology under Contract 2016YFA0202804 and
National Natural Science Foundation of China (21773067,
21533002, and 21573073).
Keywords: bioether • biofuel • levulinate • palladium • silica-
carbon composite
Catalyst characterizations
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Entry for the Table of Contents (Please choose one layout)
FULL PAPER
A facile reductive-
etherification
method over a
combination of
Pd/SiO2-C and Beta
catalysts has been
developed for
Qianqian Cui,
Yinshuang Long, Yun
Wang, Haihong Wu,
Yejun Guan*, Peng
Wu
Page No. – Page No.
preparing a bioether
based fuel additive,
ethyl-4-ethoxy
pentanoate, in high
yield.
A facile route
toward the
synthesis of ethyl-4-
ethoxy pentanoate
in high yield:
reductive
etherification of
ethyl levulinate in
ethanol on Pd/SiO2-
C catalysts
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